The field of the invention relates to filters such as filter-in-filter cartridges useful for fuel-water separation. In particular, the field relates to a filter-in-filter fuel-water separator and particulate filters preferably comprising thermoplastic material.
The subject matter of this application relates to U.S. application Ser. No. 12/820,784, filed concurrently herewith on Jun. 22, 2010, published as U.S. Published Patent Application No. 2011/0168,621, on Jul. 14, 2011, and entitled “TWO STAGE WATER SEPARATOR AND PARTICULATE FILTER”, the content of which is incorporated herein by reference in its entirety.
Coalescers are used widely to remove immiscible droplets from a gaseous or liquid continuous phase, such as in crankcase ventilation (CV) filtration, fuel water separation (FWS), and oil-water separation. Prior art coalescer designs incorporate the principles of enhanced droplet capture and coalescence by utilizing graded capture (i.e., decreasing fiber diameter, pore size and/or porosity in coalescing media) or by utilizing thick depth coalescers. Wettability also is recognized as affecting coalescer performance. (See, e.g., U.S. Pat. No. 6,767,459 and U.S. published Patent Application Nos. 2007-0131235 and 2007-0062887). U.S. Pat. No. 5,443,724 discloses that the media should have a surface energy greater than water in order to improve coalescer performance (i.e., that the media should be preferentially wetted by both coalescing droplets and continuous phases). U.S. Pat. No. 4,081,373 discloses that coalescing media should be hydrophobic in order to remove water from fuel. U.S. published Patent Application No. 2006-0242933 discloses an oil-mist coalescer in which the filtration media is oleophobic, thereby enabling the fluid mist to coalesce into drops and drain from the filtration media.
With regard to the removal of water from fuel, there is a need to increase removal efficiency and to remove smaller droplets than in the past. This challenge is further magnified by the introduction of new fuels with lower interfacial tensions and different additive packages, than fuels in the past. In particular, ultra low sulfur diesel (ULSD) fuel and biodiesel tend to have lower interfacial tensions (IFT), and therefore have smaller droplet size and more stable emulsions than previous diesel fuel. In fuels with lower interfacial tension, the size of dispersed droplets is decreased, making the droplets more difficult to remove. Enhanced coalescence therefore is needed to meet these challenges. Improved coalescers that include improved coalescing media also are desirable because they permit the use of a smaller media pack in view of improved coalescing efficiency. In fuels with lower interfacial tension, the size of droplets is decreased, making the droplets more difficult to remove.
Traditional fuel-water separators (FWS) tend to be single-stage devices designed to be used upstream of the fuel pump. In traditional FWS, the filter media is phobic with respect to the dispersed water phase and acts as a barrier. However, traditional FWS tend not to provide adequate water removal for ULSD fuel and biodiesel with low IFTs (<15 dynes/cm) and low separability (<50%) because their pore size tends to be too large to effectively capture the small droplets. As such, a large droplet size is required for effective capture. This large droplet size also is a requirement necessitated by the need to maintain the pressure drop across the FWS to well below the 1 atmosphere of pressure available when the FWS is use upstream of the fuel pump. Also, even when the mean pore size is sufficiently small, FWS media and fibrous filter media in general possess a maximum pore size so large that excessive amounts of water passes through these large pores. In modern high pressure common rail fuel systems where it is important to remove nearly all non-dissolved water from fuel passing to the injectors, the amount of water that passes through these large pores is unacceptable. Also, in modern HPCR fuel systems it is often desirable for the fuel water separator to be located on the pressure side of the pump, where the filter is exposed to higher pressures and the size of water droplets is much smaller. Traditional two-stage fuel-water coalescers (FWC) are designed to be used downstream of the fuel pump and tend to be two-stage devices for fuel in which the first stage captures the droplets, holds them so coalescence can occur, then releases the enlarged drops which are removed by sedimentation/settling, typically after being blocked by the second separator stage (where the second separator stage acts as an FWS). Traditional two-stage FWC tend to provide higher removal efficiency than FWS, but tend to have insufficient life, due to plugging by solids or semisolids. To varying degrees, both FWS and FWC are adversely affected by the presence of surfactants in fuels that lower interfacial tension, reduce droplet size, slow down the rate of coalescence, stabilize emulsions, and may adsorb onto media and render it less effective. As such, there is a need for improved fuel-water separators that exhibit a high efficiency, low pressure drop, and are minimally affected by low interfacial tension and the presence of surfactants.